EUV Range Research Articles

ON EXTREME-ULTRAVIOLET HELIUM LINE INTENSITY ENHANCEMENT FACTORS ON THE SUN

Helium lines in the solar EUV spectrum provide useful diagnostics of the solar atmosphere plasma. Helium is one of the few elements that exhibits strong emission lines formed in the lower transition region, and it is the second most abundant element in the Sun. However, the analysis of helium lines is complicated by their optical thickness and the unusual behavior of their intensities, with enhancements by a factor of up to 15 reported in the literature. Detailed study requires spatially and spectrally resolved observations in the EUV range, as well as sophisticated atomic modeling. The present work focuses on the application of the differential emission measure distribution to reproduce the observed fluxes of the He i and He ii lines observed by the Solar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer and Hinode/EIS spectrometers, using the latest atomic data. It is found that the comparison between observed and reconstructed intensities for He i resonance lines λλ537.03, 522.21, and 515.62 and the intercombination line λ591.41 does not show a real enhancement. By contrast, He i λ584.33, the first line of the resonance series, shows a depletion of a factor ~2, due to the opacity effect, as supported by non-LTE radiative transfer calculations. For single ionized helium lines 303.78 and 256.32 A, the enhancement factors obtained are higher and agree with those of previous work. The different behavior of He i and He ii lines suggests a mechanism that affects ionized helium only.

Investigating the radiation hardness of semiconductor detectors toward synchrotron radiation in the EUV range (near 13.6 nm)

The main techniques used at the COSMOS synchrotron radiation station to study the radiation hardness of semiconductor detectors are described. The spectral region around the photon energy of 100 eV is singled out from the white SR beam. A small area on the receiving detector’s surface is irradiated with a focused SR beam, and the sensitivity of the exposed and unexposed areas is investigated. The optics comprise full external reflection mirrors and thin-film filters. The techniques for calculating the surface dose are presented along with the results from measurements.

Laser plasma cryogenic target on translating substrate for generation of continuously repetitive EUV and soft X-ray pulses.

To generate continuously repetitive EUV and soft X-ray pulses with various wavelengths from laser-produced plasmas, a one-dimensionally translating substrate system with a closed He gas cryostat that can continuously supply various cryogenic targets for ~10 Hz laser pulses has been developed. The system was successfully operated at a lowest temperature of 15 K and at a maximum up-down speed of 12 mm/s. Solid Ar, Kr, and Xe layers were formed, and their growth rates and the laser crater sizes on them were studied. By optimization of the operational parameters in accordance with our design rule, it was shown that stable output power was achieved continuously from the plasma emission at frequencies of 1-10 Hz. The average soft X-ray and EUV powers obtained were 19 mW at 3.2 nm, 33 mW at 10.0 nm, and 66 mW at 10.8 nm, with 10% bandwidths, from the Ar, Kr, and Xe solid targets, respectively, with a laser power of 1 W. We will be able to achieve higher frequencies using a high beam quality laser that produces smaller craters, and can expect higher powers. Although only Ar, Kr, and Xe gases were tested in this study, the target system achieved a temperature of 15 K and can thus solidify almost all target gases, apart from H and He, and can continuously supply the solid target. The use of various target materials will enable expansion of the EUV and soft X-ray emission wavelength range.

EUV Ellipsometry on Mo/Si Multilayers

We investigate polarisation properties of a reflective Mo/Si multilayer system in the EUV range using polarized synchrotron radiation at BESSY-II. The characterization involves reflectivity measurements with s- and p-polarized light as a function of the wavelength for three different angles near normal incidence. The phase retardance is determined near normal incidence for one fixed angle of incidence as a function of the wavelength. As an additional spin-off of the polarimetry measurement the Stokes parameters of the beamline could be determined. With the 8-axis UHV-polarimeter we have measured the complex reflection coefficients for the first time and establish this ellipsometry technique as an additional sensitive probe to characterize and model multilayer optical elements.

Extreme-ultraviolet and hard X-ray signatures of electron acceleration during the failed eruption of a filament

We search for EUV brightenings in TRACE 171 {\AA} images and HXR bursts observed during failed eruptions. We expect that if an eruption is confined due to interaction with overlying magnetic structures then we should observe effects connected with reconnection between magnetic structures and acceleration of particles. We utilized TRACE observations of three well observed failed eruptions. EUV images were compared to HXR spatial distribution reconstructed from Yohkoh/HXT and RHESSI data. The EUV light curves of a selected area were compared to height profiles of eruption, HXR emission and HXR photon spectral index of power-law fit to HXR data. We have found that EUV brightenings are closely related to the eruption velocity decrease, to HXR bursts and to episodes of hardening of HXR spectra. The EUV brightened areas are observed far from the flaring structure, in footpoints of large systems of loops observed 30-60 minutes after the maximum of a flare. These are not `post-flare' loops that are also observed but at significantly lower heights. The high lying systems of loops are observed at heights equal to height, at which eruption was observed to stop. We observed HXR source spatially correlated with EUV brightening only once. For other EUV brightened areas we estimated the expected brightness of HXR sources. We find that EUV brightenings are produced due to interaction between the erupting structure with overlying loops. The interaction is strong enough to heat the system of high loops. These loops cool down and are visible in EUV range about 30-60 minutes later. The estimated brightness of HXR sources associated with EUV brightenings shows that they are too weak to be detected with present instruments. However, next generation instruments will have enough dynamic range and sensitivity to enable such observations.

Polarization properties of Mo/Si multilayers in the EUV range

We investigate polarization properties of two novel reflective Mo/Si multilayers (ML) in the EUV range using polarized synchrotron radiation at the BESSY-II storage ring facility. One of the Mo/Si ML is used as a retarder, the other one as an analyzer within the experimental setup of the BESSY soft-x-ray polarimeter. The analyzer multilayer is characterized by performing reflectivity measurements with s- and p-polarized light as a function of the incidence angle for different wavelengths. The characterization of the retarder multilayer consists of reflectivity measurements with s- and p-polarized light as a function of the wavelength for three different angles near normal incidence. In addition the phase retardance on reflection was determined for one angle of incidence as function of wavelength. Uncertainties of the phase retardance are estimated via the block bootstrap method. As an additional by-product of the ML characterization the Stokes parameters of the beamline could be determined. With the 8-axis BESSY polarimeter we have measured the complex reflection coefficients for the first time and established this ellipsometry technique as an additional probe to characterize multilayer optical elements.

A reflectometer for at-wavelength characterisation of gratings

The design for an UHV-reflectometer for XUV-radiation is presented, which is dedicated to at-wavelength characterisation on high precision gratings. At-Wavelength Metrology is a powerful and necessary characterisation tool for the development and characterisation of optical elements. Since the optical constants of the coating materials involved are dependent on wavelength, information on e.g. reflectivity can only be obtained at-wavelength and cannot be provided by ex-situ methods.In our institute a technology centre for production and characterisation of highly efficient precision gratings is established. Within this project a reflectometer for at-wavelength characterisation of the fabricated blazed gratings is developed and manufactured. This reflectometer complements the SXR-metrology instrumentation at BESSY-II: the existing reflectometer and the polarimeter/ellipsometer chamber for polarisation studies on magneto-optical samples or non-magnetic multilayers.The main feature of the reflectometer is the possibility to incorporate real gratings with a length up to 600mm, adjustable in six degrees of freedom by a custom designed tripod system. The reflectivity is measured between −180° and +180° incidence angle for both s- and p-polarisation geometry. A variety of detectors with a high dynamic range is accessible.The reflectometer is coupled permanently to the new optics beamline on a BESSY-II bending magnet operating in the UV, EUV and soft x-ray range with the polarisation adjustable to either linear or elliptical. The station will be available by the end of 2013.

Investigation of the thermal stability of Mg/Co periodic multilayers for EUV applications

We present the results of the characterization of Mg/Co periodic multilayers and their thermal stability for the EUV range. The annealing study is performed up to a temperature of 400∘C. Images obtained by scanning transmission electron microscopy and electron energy loss spectroscopy clearly show a good quality of the multilayer structure. The measurements of the EUV reflectivity around 25 nm (∼49 eV) indicate that the reflectivity decreases when the annealing temperature increases above 300∘C. X-ray emission spectroscopy is performed to determine the chemical state of the Mg atoms within the Mg/Co multilayer. Nuclear magnetic resonance used to determine the chemical state of the Co atoms and scanning electron microscopy images of cross sections of the Mg/Co multilayers reveal changes in the morphology of the stack from an annealing temperature of 305∘C. This explains the observed reflectivity loss.

Erratum to: “Dependence of Forbush-Decrease Magnitudes on Parameters of Solar Eruptions”

By data of the 23rd solar cycle, it is shown that close statistical relations exist between quantitative parameters of dimmings and arcades caused by solar coronal mass ejections (CMEs), on the one hand, and magnitudes of non-recurrent Forbush-decreases of the galactic cosmic ray flux, as well as the propagation time of disturbances from the Sun to the Earth, on the other hand. Parameters of dimmings and arcades, in particular their summarized magnetic flux of the prolonged field at the photospheric level, were calculated by data of the EUV SOHO/EIT telescope in the 195 A Received results mean that the scale, characteristics, and propagation time of interplanetary disturbances to the Earth are determined to a large degree by measurable parameters of solar eruptions and may be estimated in advance by observations of dimmings and arcades in the EUV range.

Kinematical evidence for physically different classes of large-scale coronal EUV waves

Context. Large-scale wavelike disturbances have been observed in the solar corona in the EUV range since more than a decade. The physical nature of these so-called “EIT waves” is still being debated controversially. The two main contenders are on the one hand MHD waves and/or shocks, and on the other hand magnetic reconfiguration in the framework of an expanding CME. There is a lot of observational evidence backing either one or the other scenario, and no single model has been able to reproduce all observational constraints, which are partly even contradictory. This suggests that there may actually exist different classes of coronal waves that are caused by distinct physical processes. Then, the problems in interpreting coronal waves would be mainly caused by mixing together different physical processes. Aims. We search for evidence for physically different classes of large-scale coronal EUV waves. Methods. Kinematics is the most important characteristic of any moving disturbance, hence we focus on this aspect of coronal waves. Identifying distinct event classes requires a large event sample, which is up to now only available from SOHO/EIT. We analyze the kinematics of a sample of 176 EIT waves. In order to check if the results are severely affected by the low cadence of EIT, we complement this with high-cadence data for 17 events from STEREO/EUVI. In particular, we focus on the wave speeds and their evolution. Results. Based on their kinematical behavior, we find evidence for three distinct populations of coronal EUV waves: initially fast waves (v ≥ 320 km s −1 ) that show pronounced deceleration (class 1 events), waves with moderate (v ≈ 170−320 km s −1 ) and nearly constant speeds (class 2), and slow waves (v ≤ 130 km s −1 ) showing a rather erratic behavior (class 3). Conclusions. The kinematical behavior of the fast decelerating disturbances is consistent with nonlinear large-amplitude waves or shocks that propagate faster than the ambient fast-mode speed and subsequently slow down due to decreasing amplitude. The waves with moderate speeds are consistent with linear waves moving at the local fast-mode speed. Thus both populations can be explained in terms of the wave/shock model. The slow perturbations with erratic behavior, on the other hand, are not consistent with this scenario. These disturbances could well be due to magnetic reconfiguration.

Estimation of the XUV radiation onto close planets and their evaporation

Context: The current distribution of planet mass vs. incident stellar X-ray flux supports the idea that photoevaporation of the atmosphere may take place in close-in planets. Integrated effects have to be accounted for. A proper calculation of the mass loss rate due to photoevaporation requires to estimate the total irradiation from the whole XUV range. Aims: The purpose of this paper is to extend the analysis of the photoevaporation in planetary atmospheres from the accessible X-rays to the mostly unobserved EUV range by using the coronal models of stars to calculate the EUV contribution to the stellar spectra. The mass evolution of planets can be traced assuming that thermal losses dominate the mass loss of their atmospheres. Methods: We determine coronal models for 82 stars with exoplanets that have X-ray observations available. Then a synthetic spectrum is produced for the whole XUV range (~1-912 {\AA}). The determination of the EUV stellar flux, calibrated with real EUV data, allows us to calculate the accumulated effects of the XUV irradiation on the planet atmosphere with time, as well as the mass evolution for planets with known density. Results: We calibrate for the first time a relation of the EUV luminosity with stellar age valid for late-type stars. In a sample of 109 exoplanets, few planets with masses larger than ~1.5 Mj receive high XUV flux, suggesting that intense photoevaporation takes place in a short period of time, as previously found in X-rays. The scenario is also consistent with the observed distribution of planet masses with density. The accumulated effects of photoevaporation over time indicate that HD 209458b may have lost 0.2 Mj since an age of 20 Myr. Conclusions: Coronal radiation produces rapid photoevaporation of the atmospheres of planets close to young late-type stars. More complex models are needed to explain fully the observations.

EUV soft X-ray characterization of a FEL multilayer optics damaged by multiple shot laser beam

We have investigated the damaging effects of a femtosecond pulsed laser beam with 400 nm wavelength on a Mo/Si EUV multilayer. The exposures have been done in vacuum with multiple pulses (5 pulses/mm 2) of 120 fs varying the laser fluence in the 38–195 mJ/cm 2 range. The analysis of the different irradiated regions has been performed ex-situ by means of different techniques, including specular and diffuse reflectivity, X-ray photoemission spectroscopy (XPS) and total electron yield (TEY) in the EUV and soft X-ray range. Surface images have been acquired by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Results clearly indicate a progressive degradation of the EUV multilayer performances with the increase of the laser fluence. Spectroscopic analysis allowed to correlate the decrease of reflectivity with the degradation of the multilayer stacking, ascribed to Mo–Si intermixing at the Mo/Si interfaces of the first layers, close to the surface of the mirror.

Core Impurity Transport Studies from Different Diagnostic Approaches in LHD

Impurity transport has been studied in the Large Helical Device (LHD) with different diagnostic approaches based on an active method that combine carbon pellet injection with visible bremsstrahlung measurement and three passive methods for radial profile measurements of Ar and Fe Kα X-ray lines, Zeff, and extreme ultraviolet (EUV, ≤500 Å) impurity line emissions, in addition to usual passive spectroscopy. The existence of an inward convective velocity is confirmed in the edge region (ρ > 0.6) using the active method, whereas no convection is required in the core region (ρ < 0.6). The electron density dependence is weak for the diffusion coefficient (typically D = 0.15 to 0.25 m2/s) for densities of 1 to 5 × 1013 cm-3 but is strong for the inward convective velocity, which varies in the range of V(a) = −0.2 to −1.5 m/s. The inward V in helium plasmas (−0.4 m/s at ρ = 0.8 and the central density, ne ~ 4.0 × 1013 cm−3) is nearly half that in hydrogen plasmas (−0.7 m/s). This difference suggests a charge state dependence of fuel ions predicted by the neoclassical theory. Radial profiles of impurity transport coefficients of argon and iron have been studied using spatially resolved soft X-ray pulse-height analyzers. The impurity transport has also been studied in extremely high density discharges achieved by H2 pellet injection based on the passive spectroscopy and Zeff profile measurement. A flat Zeff profile is obtained at ne = 2.5 × 1014 cm−3 with values of 1.1 ≤ Zeff ≤ 1.2, suggesting no existence of impurity accumulation and radially constant impurity partial pressure. Finally, radial profiles of impurity lines in the EUV range are analyzed with the transport coefficients.

Development and Interfacial Characterization of Co/Mg Periodic Multilayers for the EUV Range

We propose a new system, namely the periodic Co/Mg multilayer system, for optics applications in the EUV range. Close to the Mg L edge, i.e., around a wavelength of 25 nm or a photon energy of 50 eV, a reflectivity of about 43% is measured at 45° for s-polarized radiation. Moreover, it appears that this system is stable over a period of time of three months. The introduction of thin boron carbide interfacial layers proves disastrous contrary to simulations that show this could be beneficial. We combine X-ray reflectivity in the hard X-ray range, X-ray emission spectroscopy, and nuclear magnetic resonance to determine the thickness and roughness of the Co and Mg layers as well as the chemical state of the Co and Mg atoms at the interfaces. This reveals that in the Co/Mg system the interfaces are abrupt and there is no interdiffusion between the Co and Mg layers. Then the difference between the experimental and simulated reflectivities is ascribed to the interfacial roughness of the order of 0.4 nm. In the Co/Mg/B 4 C system, evidence of a large mixing of the Co and B 4 C layers is presented and explains the poor reflectance of this system.

X-exoplanets: an X-ray and EUV database for exoplanets

AbstractExtreme Ultraviolet (EUV) and X-ray emission is of great importance in several phenomena related to the formation of planetary systems and the atmospheres of planets. The atmospheric composition, and the mass of an exoplanet, are partly dependent on the X-ray and EUV radiation received during the first stages of formation and even during main sequence of the star. Biological life developing on exoplanets would depend severely on the high energy radiation arriving from its parent star.Here we present a database of the X-ray and EUV emission of all the stars currently known to host exoplanets. The archive is public and accessible through the Spanish Virtual Observatory (SVO). The database gives the user the option to download observed X-rays and EUV spectra. Synthetic spectra covering the spectral range 1–912 Å are also available (present day telescopes do not give access to the EUV range at λ &gt; 180 Å). These spectra are created using coronal models after fitting observed spectra.

Dynamics of plasmoids formed by the current sheet tearing

Context.Moving blob-like features observed in the soft X-ray and EUV range above flare-loops are often interpreted as signatures of plasmoids formed by the current sheet tearing in the flare-associated reconnection process.

Possibility of population inversion of Fe XVII ions in dense plasma conditions

We present the possibility of population inversion in 3p–3s quantum transitions of Ne-like Fe ions and the consequent laser action in the EUV range in the Centaurus X-3 (Cen X-3) x-ray pulsar. The effect of bulk motions of the ions on the laser gain is considered. The hydrodynamic Doppler broadened gain coefficients for the six lines of Ne-like Fe at 25.5 nm, 20.6 nm, 34.1 nm, 35.6 nm, 38.4 nm and 38.5 nm are calculated. The plasma velocity affects the laser gain by an order of magnitude 3. The gain coefficients are significant in the region of electron density 1018 < ne < 1022 cm−3 and electron temperature Te ⋍ 250–1000 eV. The lines at 25.5 nm and 20.6 nm have large linear amplification coefficient which implies that stimulated emission is possible in these lines under the physical conditions of the Cen X-3.

On the possibility of applying kinoform elements in mirror projectors for EUV lithography

The potential of the technology of reflective diffraction (holographic) optical elements intended for the EUV range is considered, and their application in projection lithography is discussed.

Underdense radiation sources: Moving towards longer wavelengths

Underdense radiation sources have been developed to provide efficient laboratory multi-keV radiation sources for radiography and radiation hardening studies. In these plasmas laser absorption by inverse bremsstrahlung leads to high x-ray conversion efficiency because of efficient ionization of the low density aerogel or gas targets. Now we performing experiments in the soft x-ray energy regime where the atomic physics models are much more complicated. In recent experiments at the NIKE laser, we have irradiated a Ti-doped SiO 2 aerogel with up to 1650J of 248 nm wavelength light. The absolute Ti L-shell emission in the 200-800 eV range is measured with a diagnostic that uses a transmission grating coupled to Si photodiodes. We will give an overview of the temporally-resolved absolutely calibrated spectra obtained over a range of conditions. Eventually we hope to extend our studies to x-ray production in the EUV range.

Multi-wavelength study of a high velocity event near a sunspot

Context. Jets with velocities higher than 200 km s 1 are often observed in the X-ray spectral range in the solar atmosphere, however, very few such objects are observed in the EUV range. Aims. We report on an eruptive event observed in the south-west side of a sunspot, within active region, AR0554, in a range of spectral lines formed in the transition region and corona. Methods. The main data were time-series observations obtained with the Coronal Diagnostic Spectrometer (CDS), in addition to images from the Michelson Doppler Imager (MDI) and Extreme ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory. Additional high resolution images from the Transition Region and Coronal Explorer (TRACE) were also used. Results. The event brightened a loop connecting the sunspot and a bright-point structure, triggering both a high-speed o w in the loop and a transient, low-speed ambient o w into the sunspot. The energy released is quickly conducted to the chromosphere, as deduced from the TRACE 1600 ¯ images and the light curve of Hei 522 ¯ as seen by CDS. The results from our multi-wavelength study further indicates that the e ect of the eruptive event may have reached temperatures higher than 2 10 6 K. Relative Doppler velocities of over 300 km s 1 were detected in the transition region line, Ov 629¯. Conclusions. The multi-wavelength analysis indicates that the event is likely due to fast magnetic reconnection in the transition region. The velocities detected are consistent with the results obtained by the evaporation jet model.